Direct current amplifier



Novf 7, 1939. D. G c. LUCK DIRECT CURRENT AMPLIFIER Filed Aug. 14, 19372 Sheets-Sheet l Nov. 7, 1939. D. G c. LUCK DIRECT CURRENT AMPLIFIERFiled Aug. 14, 1957 2 Sheets-Sheet 2 (Ittomeg Patented Nov. 7, 1939UNITED STATES DIRECT CURRENT AMPLIFIER David G. C. Luck, Haddon Heights,N. l, assignor to Radio Corporation of America, a corporation ofDelaware Application August 14, 1937, Serial No. 159,025

3 Claims.

The present invention relates to a direct current amplifier embodying aplurality of electric discharge amplifier tubes responsive to an applieddirect current variation at the input circuit to provide a correspondingchange in the output voltage or current.

It is an object of the present invention to pro vide an amplifier towhich an alternating potential may be applied and controlled by a directJ Voltage of relatively low value whereby, in effect, amplification ofthe direct control voltage by an alternating current amplifier ispermitted, thereby avoiding the instability common to direct currentamplifiers.

It is also an object of the present invention to provide an electricdischarge tube amplifier responsive to relatively small variations in adirect voltage applied thereto to control an alternating current inphase and magnitude, whereby the amplifier may be utilized for thecontrol of the operation of alternating current motors and the like.

Other objects and advantages of the invention will be apparent from thefollowing description when considered in connection with theaccompanying drawings in which Figure 1 is a schematic circuit diagramof an amplifier embodying the invention, and

Figures 2 and 3 are similar schematic circuit diagrams of modificationsof the circuit of Fig. 1.

Referring to Fig. 1, a detector stage 5, utilized as a source ofcontrolling potential, is provided with input leads 6 and l connectedrespectively with the control grid 8 and the grounded cathode 9 of thetube 5. The output anode I ii is connected through a series outputresistor l I with a positive anode potential supply lead l2. The anodeis also connected to ground through a suitable filter capacitor 13.Signals other than direct current variations are taken from the anodethrough a series resistor 44 and output coupling capacitor it for anysuitable utilization device (not shown).

The detector stage 5 is primarily utilized in controlling a main directcurrent amplifier included within the dash line rectangle l6 andcomprising at the input end a pair of pentode electric dischargeamplifier devices ll and it, each having a cathode I9, a control grid20a or 20b, a screen grid 2 l, a suppressor grid 22 and an output anode23.

The control grid 20a is connected to the control potential source and isbiased to a degree controlled by the resistor 39. The control grid 20bis grounded. The output anodes are con nected in push-pull or balancedrelation to each other and coupled to a pair of balanced or pushpullamplifier tubes 25 and 26.

On the input side, the control grid Ella of the tube H is connectedthrough a series isolating resistor 2'! with a variable source ofnegative biasing potential comprising the movable contact 23 of aresistor element 253 connected across a battery or other suitable directcurrent potential source 3! interposed in the output circuit of thedetector 5. An input terminal 38 is connected with the ground lead l,with the cathode 9 of the detector and with the cathodes it of theamplifier devices ll and it through a self bias resistor 39. The controlgrid 20b of the amplifier I8 is also connected to the ground lead lthrough a lead 40 and the control grid Ziia of the amplifier tube H isgrounded through a by-pass capacitor M.

It will, therefore, be seen that both control grids of the balancedamplifiers ill and it may receive the same biasing potential, the tube58 receiving the bias directly from ground, the tube ll receiving thebias through the input circuit 3836-3l-292l. The connection for the grid20a of the amplifier tube ll is directly through conductive connectionwith the anode it of the stage 5 and is, therefore, subjected to theanode potential. This connection is provided to permit adjustment of thevalue of the unidirectional control across leads 3i33i3 to a value forwhich zero output is desired, This biasing arrangement permits thebiasing potential on the two tubes to be equalized by adjusting thebiasing potential on the tube ll with respect to the fixed biasingpotential on the tube 18.

The suppressor grids 22 are connected to the cathodes Hi. The outputanodes 23 are coupled to the tubes 25 and 26 by a balanced or push-pullcoupling means admitting of relatively high gain. In the presentexample, this coupling means is of the impedance or resistance couplingtype comprising anode coupling resistors connected in balanced relationto the output anodes 23 and having a common anode potential supply leadt3 which receives positive anode operating potential from a positivesupply lead id through a series voltage reducing resistor 45. A suitableby-pass capacitor 46 is provided for the lead it? to ground 7. The gridsof the tubes 25 and 26, indicated at 41 and 38, respectively, arecoupled to the output resistors 42 through coupling capacitors 49, whichare connected to grid resistors 5E3 having a common return lead 5! toground and to the cathodes 52 through a self bias resistor 53. The tubes25 receive anode potential from the anode supply 44 through a seriesresistor 54, a common supply lead 55 and the push-pull or balancedprimary 56 of a coupling transformer 571 to which primary winding theanodes 58 are connected.

The transformer 51 is provided with a pushpull or balanced secondarywinding 59 to which the grids ti of a pair of power amplifier tubes ti!and 62 are connected in balanced or push-pull relation. The tubes 6! and52 are also provided with a balanced or push-pull output transformer $3having a balanced primary winding 64 connected with the anodes 65 andwith the positive potential supply lead 44 through a common lead 66.Each half of the primary winding is provided with a spark gap indicatedat 61 for preventing excessive potential surges across the primarywinding. The input secondary winding is connected at the center tap 68directly with the cathodes 89 and the ground lead 1, the latterconnection being indicated by the lead 10, this connection being madethrough capacitors 68a and 88b for the purpose of suppressinginterference from a motor '33 which is controlled through the circuitspreviously described.

The secondary H of the output transformer 63 is provided with outputleads [2 connected with the armature of the alternating current motor iswhich is provided with a field winding l4 connected through supply leadsI5 and a phase controlling capacitor 16 and resistor 11 with analternating current supply source indicated by the leads E8. The supplyleads T8 are isolated from the motor leads 15 by an isolatingtransformer 79 which may step up or step down the alternating voltage asmay be required for the motor winding 14, the proper phase beingdetermined by the capacitor 16 and resistor 'l'l.

Alternating current energy from the secondary winding of the isolatingtransformer is applied to the motor element 13 to operate it in eitherdirection. Thus, alternating current energy is applied to the balancedcontrol stage of the amplifier comprising the tubes I1 and I8 on thescreen grids 2i through a supply lead 8! which is connected to thealternating current transformer T9 to derive a suitable potentialtherefrom with respect to the cathode return or ground lead 1 which isextended to connect with one side of the alternating current secondary,as indicated at 3!. A predetermined potential is derived with respect tothe terminal 8|, for the lead St, by a suitable tap 82 on apotentiometer resistor 83 connected across the secondary of thetransformer 19, thereby applying the alternating potential existingbetween the tap 82 and the terminal 8! across a portion of the resistor83, between the ground lead i or cathode return and the screen grids 2!.This potential is so chosen that when applied through the successiveamplifier stages it results in a normal operating voltage at the outputleads 12 for the full operation of the motor.

The alternating current is thus introduced or coupled electronically tothe push-pull or balanced amplifier channel at the input or controlstage, the input or control grids of which are normally balanced toprovide zero output. The control grids 28 of the control stage arenormally biased to a region of considerable curvature of the tubecharacteristics. The direct control voltage from the controlpotentiometer 28Z9 is applied to the one control grid. For one controlvoltage value corresponding to the fixed bias provided on the controlgrid 26b of the tube 18, the tubes are thus balanced and no outputoccurs.

If the control potential is now varied in a positive or negativedirection, the potential on the control grid of the tube ll willincrease or decrease with respect to the voltage on the control grid ofthe static tube l8, thereby causing the gain of the tube H to decreasewith increased negative bias and to increase with decreased negativebias with respect to the gain of the static tube l8 and thereby causingthe alternating potential to be amplified in the amplifier stage 25-26and in the power output stage iii-62 and to be applied through the leads72 to the motor element i3. Departure from the balanced condition of thecontrol stage thus produces a control potential which is utilized toproduce an output voltage corresponding to the direction of departurefrom balance as one half or the other of the balanced amplifier channelis caused to have the higher gain. With this arrangement, thealternating current motor 13-44 may be caused to operate in eitherdirection with increasing power or speed as the contact 28 of thecontrol potentiometer is adjusted in either direction from the conditionof balance or as the control potential causes the condition of balanceto be varied in either direction on the control amplifier ll.

Sudden or abrupt changes in potential producing a transient voltage waveof any appreciable magnitude are reduced by the filter 2l-4l. However,as a precaution against surges, the power amplifier is provided with theflash-over gap across each half of the output transformer primary ashereinbefore described.

The amplifier output stage must be capable of transmitting the requiredamount of power for operation of the motor. The preceding stage orstages represented by the voltage amplifier stage 2?; 2%; must becapable of providing sufficient gain for operating the power outputstage and the input or control stage must be adapted to provide apush-pull or balanced output with a parallel connected input circuit forthe control grids whereby the input amplifiers or control stage may bebrought to a condition of static balance so that the control potentialmay be effective to control the operation of the motor as previouslyindicated. The control source driving the balanced input stage must, ofcourse, provide a sufficiently wide range of control potential toincrease the biasing potential on the control tube H to provide fullgain through onehalf of the amplifier channel and to decrease thebiasing potential suificiently to provide full gain through the oppositehalf of the amplifier channel whereby the motor is caused to operatewith predetermined power rating and speed in either direction.

The system of Fig. 2 is similar to that of Fig. l in some respects, butdiifers therefrom in that the alternating voltage is applied to a pairof control grids instead of the screen grids as in Fig. 1. Zero outputof the control tubes H--l8 is established by means of a bias potentialsource 90 across which is connected a resistor 91 for biasing thecontrol grid of the device l l and a resistor 92 for biasing the controlgrid of the device l8. With these connections, the potentiometer Qi isadjusted for operation. on the most curved part of the tubecharacteristic with the normal value of direct current input, and thepotentiometer 92 is adjusted to produce zero alternating current outputfor normal direct current input. The

operation of this system is substantially the same as that of Fig. 1.

The system of Fig. 3 differs from that of Fig. l in that the alternatingcurrent is introduced into the plate circuit of the control tubes 93 and94. This system, like those previously described operates to control thealternating current output in response to variation in theunidirectional input or control voltage.

From the foregoing description, it will be seen that through the mediumof a balanced direct current control stage having the input gridsconnected in parallel and the output anodes connected in push-pullrelation, a relatively low alternating current potential may beintroduced electronically into the control stage on the screen grids ofa pair of pentodes constituting such stage and that relatively smallvariations in direct controlling potential may be caused to provideeffective control of the alternating current output of the amplifierchannel, both in magnitude and in phase, thereby to provide analternating current control potential adapted to operate alternatingcurrent apparatus such as motors and the like at controllable levels andin either direction.

I claim as my invention:

1. A direct current controlled alternating current amplifier including apair of electron discharge devices provided with push-pull connectedoutput circuits and with parallel connected pairs of input circuits,means for applying alternating potential to one of said pairs of inputcircuits, means for applying a unidirectional control potential to oneof the other pair of said input circuits, and means for adjusting saidunidirectional control potential to a predetermined value at which noalternating current is produced in said output circuits wherebyvariations of said control potential from said value varies thealternating current delivered from said output circuits.

2. In the operation of a direct current controlled alternating currentamplifier including a pair of electron discharge devices provided withpush-pull connected output circuits and with pairs of parallel connectedinput circuits, the method which comprises applying alternatingpotential to one of said input circuit pairs, and applying to one inputcircuit of said other pair a unidirectional control potential andadjusting said unidirectional control potential to a normal value atwhich no alternating current is produced in said output circuit, wherebyvariations of said control potential from said value in oppositedirections reverses the phase of the alter nating current delivered fromsaid output circuit.

3. The combination of a pair of electron discharge devices provided withpush-pull connected output circuits and with a plurality of input gridcircuits, means including an alternating current connection to two ofsaid grid circuits in parallel for producing alternating current in saidoutput circuits, means for applying bias potentials to another of saidinput circuits, and means for applying to one of said input circuits aunidirectional control potential whereby the alternating currentdelivered from said output circuits may be controlled as to phase andlevel.

DAVID G. C. LUCK.

